I really don't believe the Koran is an authentic expression of God's Word. I really don't even think that the Prophet Muhammed is a religious figure to be much admired.
Yet with that said neither I, nor anybody else I know, goes around depicting a religious figure held dear to Muslims in a derogatory way. There are basic 'fundamentals' in life like 'treat others the way you would like to be treated' that, for instance, stop me from saying to an Israeli, "Oh, you're from Israel! That rogue state that murders women and children! You can grab all the land you like mate, but its sad for you guys that you missed the boat salvation-wise! You should get yourself baptised mate and at least give yourself a fighting chance of Heaven. After all, the Son of God did say not to leave Earth without it."
Equally, on encountering a Muslim, I tend not to go up to him and say, "Hello. Muslim, eh? You're so called 'prophet' is a fraud who denied Christ and spawned a rival to the One True Faith, outside of which is no salvation. You think God can't become a man? Hello? Doesn't God's omnipotence mean He can do what He likes, eh? What do you think of that, eh? How much do I owe you for the kebab? Yes, garlic mayo and chilli sauce, please..."
Yet, apparently, in the new era of 'free speech' I can behave like this and everyone is fine with that because I've 'said my piece' and offended those religions I think are either a) redundant or b) erroneous. Well, even if everyone was fine with that, which they are not now, nor ever will be, I wouldn't be fine with that, and making ardent claims towards my rights to 'free speech' wouldn't stop me being validly described as a prick of formidable magnitude with only feint respect for Almighty God and no respect for my fellow men.
Natural reason would inform my conscience that behaving like that is wrong. Faith would point to at least some cultivation of prudence in how one approaches those of other faiths. Kindness, patience, humility, charity, all would point away from treating people with that kind of sneering contempt. Of course, none of these things are beyond the mind of the secularists. Its a shame not just for them, but for the whole world, when these things are not put into practice, but are instead abandoned in order to pursue an agenda that seeks to alienate others, disparage others and mock those of religious faith. Religions, whether you think they are right, or wrong, are held incredibly dear to their adherents. As Pope Francis says, you can only insult someone's mother so many times before he punches you. If he punches you and you get up and do it again, you are an idiot. That said, as a commenter has reminded me, Our Lord told us to turn the other cheek.